Ultrasonography Of The Kidney And The Renal Vessels
Ultrasonography of the kidney and the renal vesselsPart I: Normal findings, inherited and renoparenchymatous diseasesProf. Dr. med. Jörg RadermacherDept. of NephrologyKlinikum MindenFriedrichstrasse 1732427 MindenTel: 0571/ 801 – 3021FAX: 0571/ 801 – 3076E-Mail: Joerg.radermacher@klinikum-minden.deAbstractRenal ultrasonography has become the standard imaging modality in the investigation ofkidneys because it offers excellent anatomic detail, requires no special preparation of patientsis readily available and does not expose the patient to radiation or contrast agents.Ultrasonography is used to determine the site and size of the kidney and to detect focal lesionslike tumors, cysts and renal stones. Furthermore the presence and urodynamic relevance ofhydronephrosis can reliably be found.The presence of renoparenchymatous disease as such is also discernible to the experiencedinvestigator, however most glomerular diseases cannot be further sub classified. Exceptionsare primarily renovascular disorders like hypertensive nephrosclerosis, diabetic nephropathyor renal vasculitis which can be suspected if the intrarenal resistance index value is increased.Color Doppler sonography in experienced hand allows the reliable detection andquantification of renal artery stenosis and increased resistance index values may indicateirreversible disease.Ultrasonography has also been found of value in the evaluation of renal transplant kidneys.Especially in the early transplant course potentially fatal but reversible diseases like renal veinthrombosis or urinomas are detected with high sensitivity. In the long term course anincreased resistance index value may also predict allograft failure.IntroductionRenal ultrasonography has become the standard imaging modality in the investigation ofkidneys. Renal size and location can be determined. Solid tumors can be detected and can bedistinguished from renal cysts. Ultrasonography can detect nephrolithiasis andhydronephrosis. Dilated ureters can frequently be followed up to the location of the occludingconcrement. Different renoparenchymatous diseases, especially the glomerulonephriticdiseases cannot be differentiated be ultrasonography. However, the experienced investigatorcan almost always distinguish normal from diseased kidneys. Detection of renal arteries isreliably possible with Color Doppler sonography. A separate discussion of B-mode and ColorDoppler sonography does not always make sense, because frequently only the combined useof both techniques (urinary tract obstruction, renal tumors, renal cystic disease, diabeticnephropathy ) enables the investigator to make a diagnosis. The first part of this reviewarticle will focus on normal findings and renoparenchymatous disease. The second part will
deal with focal findings including renal tumors. For more in depth information the reader isreferred to books and review articles. 1-6Patient preparation and ultrasonographic normal findingsPreparation of the patient (fasting, carminatives) is rarely necessary. The investigation shouldbe performed in a warm, dark room with the patient lying on his back. For relaxation of theabdominal rectus muscle the investigation should be performed with the upper body elevatedby 30 and possible with slightly bend knees (knee roll). This will cause a slightly more subcostal position of the kidneys which facilitates the investigation. In rare cases placement ofthe patient on his left or right side may become necessary to overcome poor visualization dueto interposition of bowl or to have a better view on the origin of the renal arteries. Changingpatient position however, may greatly prolong investigation time, especially in older patients.A primary approach with the patient in the prone position is not advised – unless one wants toperform a renal biopsy - because of poorer kidney visualization through the psoas muscle. Incase of renal biopsy a more dorsal position of the kidneys is achieved by placing the patienton a bed roll. A 2-5 MHz convex array scanner is usually used in adult sonography. Scannerswith higher frequencies have a lesser penetration depth but allow better visualization ofkidney stones. Massive meteorism may be a lesser problem with the use of a sector scanner.Harmonic imaging modalities, which are available in most modern machines, allow bettervisualization of kidneys (see below). If available a Color Doppler machine with at least a pwDoppler should be used.Figure 1: Better visualization of the right kidney with harmonic imaging and B-mode contrastenhancement (photopic)Examination procedure: To compare kidney with liver echogenicity a transhepatic depictionof the right kidney from a ventrolateral approach is advisable. The caudal pole of the kidneyhowever is frequently not visible due to interposing bowl gas. Depiction of the whole kidneyin longitudinal and transverse sections is usually possible from a dorsolateral sub costalapproach. A valsalva maneuver will frequently be necessary to see the upper pole of thekidney. Mobility of a normal kidney should be 3-7 cm. Since deep inspiration may lead toelongation of kidneys with a concomitant reduction in depth and width all dimensions should
be measured in the same breath hold position. Otherwise false renal volume calculations mayresult.Normal findingsFig. 2 Normal kidneyRenal shape and -positionThe kidneys are located retroperitoneally and slide on the musculus quadratus lumborum andmusculus psoas during in- and expiration. Due to loin lordosis the lower pole of both kidneysis located more ventrally than the upper pole. The upper pole of the right kidney is transversedby the 12th and the upper pole of the left kidney by the 11th rib. This means, the right kidneyis usually positioned 1 to 2 cm more caudally than the left kidney. The right kidney surface isadjacent to the lower liver boarder, so the liver can be used as an ultrasound window for theright kidney. The kidneys are shaped like beans, convex on the lateral und concave on themedial sight.Renal surface and capsuleThe kidneys are enclosed by an adipose capsule the thickness of which varies depending onthe general constitution of the patient. This adipose capsule can have high or low echogenicity
and may be mistaken for the renal parenchyma in patients with shrunken kidneys. However,the missing mobility during in- and expiration usually allows clear differentiation ofsurrounding fatty tissue and renal parenchyma. Kidneys usually have a smooth surface. Anormal variant are renal renculi. These are the remaining signs of fetal lobulation and aremore easily recognizable in the right kidney. Two renculi always surround a medullarypyramid.Figure 3. Persisting fetal lobulation of the right kidney (renculation)Renal parenchymaAn inexperienced investigator should assess the renal parenchyma only in comparison to theadjacent liver and spleen. The normal renal parenchyma in children age 6 and older and inadults should be slightly less echogenic than that of liver and spleen. Normal renalparenchyma from birth until 6 months of age is slightly brighter than that of the liver.Increased echogenicity compared to liver and spleen in adults is a sensitive but unspecificsign of renal disease. The normal parenchymal width of 15-25 mm can be measured mostreliably from the basis of a medullary pyramid to the kidneys surface. The renal hilum shouldbe visible when measuring parenchymal width to avoid underestimation. If such a depiction isnot technically possible a normal parenchymal width can be assumed if the parenchymapelvis relation is normal. The relation of ventral and dorsal parenchymal width to pelvic widthshould be 2 : 1. Medullary pyramids are usually less echogenic than the surroundingparenchyma. Columns of Bertini (columnae renales) are extensions of the renal parenchymain the renal pelvis and hypertrophied columns of Bertini should not be confused with pelvictumors. The left kidney sometimes has a very wide parenchyma in the medial portion, a socalled splenic notch or dromedary hump. This also is a normal finding and should not beconfused with renal masses. The echogenicity in the area of the hump will be the same as inthe surrounding parenchyma and the medullary pyramids will still be present and the vesselarchitecture will be unaffected.
Abb. 4 Splenic notch (dromedary hump), P denotes medullary pyramidsRenal sinusThe central echogenic part of the kidney (sinus) is composed of the pelvis and the calyces, ofblood and lymphoid vessels and interposed adipose tissue. The normal renal pelvis should notbe fluid filled except in pregnant women. Anechoic areas are frequently due to dilated veinsas can be easily proven by Color Doppler ultrasound. The ureter is located dorsally to therenal vessels and also should not be visible normally.Renal vesselsThe renal artery usually branches within the renal sinus or extrarenally in 2 to 3 segmentalarteries of first order and these 1st order segmental arteries branch another 2 to 3 times intosegmental arteries of 2nd and 3rd order. When entering the renal parenchyma the vessels arecalled interlobar arteries. From these the arcuate arteries branch of in a 90 angle, runningparallel to the kidney surface and the interlobular arteries, which run towards the renalcapsule, originate from the arcuate arteries. The right renal vein is relatively short (4 cm) andrarely visible from a ventral approach due to overlying bowl gas. The left renal vein runsbetween aorta and superior mesenteric artery and can rarely be compressed between these twovessels (so called nutcracker phenomenon). A retro aortal course of the left renal vein is alsopossible.Estimation of normal renal sizeChronic renal disease frequently leads to renal shrinking. Reliable criteria depicting ashrunken or enlarged kidney have not been published for adults. A renal length of 9-12 cm isconsidered normal, renal length correlating to body length. The correlation is poor however.Frequently the right kidney is shorter than the left kidney whereas renal function estimated byszintigraphy and renal volume estimated by CT are equal. A better correlation can be foundbetween renal volume and body weight or body surface area. Renal volume is frequentlyestimated as length x depth x width (cm) / 2. In children with healthy kidneys normal renalvolume (ml) could be estimated as body weight (kg) x 2. The few available data in adult
populations – which were not evaluated for normal renal function - do not contradict thesefindings. A normal renal volume can therefore be defined as body weight (kg) x 2 20%.Congenital renal diseasesRenal agenesis or hypoplasia: Usually a chance finding. Unilateral small or missing kidneywith normal architecture and echogenicity. Color Doppler and Doppler ultrasonography alsoshow normal findings. The contralateral kidney shows compensatory hypertrophy. Caveat:Unilateral small kidneys due to renal artery stenosis will also results in – age dependant –hypertrophy of the contralateral kidney. Here altered Doppler signals in the small kidney willlead to the correct diagnosis. Renal agenesis cannot be proven by ultrasonography (seeectopy).Ectopic kidneys: Usually small and malrotated. Located between the urinary bladder and thenormal position. In crossed ectopy both kidneys are located on the same side and arefrequently fused and therefore enlarged. Frequently two or more renal pelvices are present.Sometimes the kidneys can only be found with CT or MRT.Horseshoe kidney: Fusion of both lower kidney poles with a parenchymal or connectivetissue bridge (isthmus) ventral to the aorta. Here the fused part may be mistaken for enlargedlymph nodes. The lower poles of the kidneys are misplaced medially. This malformation isfrequently accompanied by vesicoureteral reflux, stone formation and urinary tractobstruction.Double kidney: Most frequent malformation (incidence 0,5-10%). B-mode ultrasonographyshows a parenchymal bridge which completely separates the cranial from the caudal part ofthe kidney. Frequently these kidneys are enlarged compared to the contralateral kidney. Theparenchymal bridge can be confused with hypertrophied columns of Bertini. Ultrasonographycannot prove the presence of double kidneys; this can only be done by an iv-urogrammshowing two renal pelvices and two ureters. Reflux or renal pelvis obstruction occurs morefrequently in double kidneys. Urinary tract obstruction may lead to dilatation of the renalpelvices in which case a double kidney can be proven by ultrasonography alone.
Figure 5: Enlarged left kidney with a parenchymal bridge which completely traverses therenal sinus. Due to crossed ectopy a further small renal parenchymal area and sinus can beseen in the lower third. This is the hypoplastic fused part of the right kidney.
Renoparenchymatous diseasesRenoparenchymatous diseases can be classified as uni- or bilateral diseases with small orlarge kidneys (see table 1). The method to evaluate kidney size as normal or not has beenshown above. A general rule is that small kidneys depict chronic renal disease and enlarged orat least normal sized kidneys depict acute and therefore potentially reversible disease. Afurther distinguishing feature is parenchymal echogenicity. Diseased kidneys in general showincreased echogenicity, however in the early stages echogenicity may still be normal.Increasing echogenicity is directly correlated to histopathological findings like globalsclerosis, tubular atrophy, leukocyte infiltration and the number of hyaline casts perglomerulum. Decreased echogenicity is correlated to the magnitude of interstitial edema 7, 8.The normal renal parenchyma should be slightly less echogenic than that of the liver. This isproblematic insofar, that it assumes liver parenchyma to be of normal echogenicity. Manypatients, however, have fatty liver disease. Objective and easily applicable methods forquantification of renal parenchymal echogenicity are missing. Therefore the classification ofrenal echogenicity depends to a large amount on investigator experience. A further criterionseparating normal from abnormal kidneys is renal perfusion. The renal resistive index (RI decrease of minimal diastolic in relation to maximal systolic Doppler flow velocity, e.g. 0.6 60% decrease) has been best evaluated so far. The RI is increased in hypertensivenephrosclerosis 4 and correlates with the histological severity of glomerulosclerosis andarterio- and arteriolosclerosis. Table 1 summarizes the major diseases associated with small orenlarged kidneys.Figure 6: Right kidney with increased echogenicity as a sensitive but unspecific sign of renaldisease. Compare the echogenicity of the kidney with that of the overlying liverDiabetic nephropathyDiabetic nephropathy is the most frequent renoparenchymatous disease, accounting for 40%of cases of incident terminal renal failure. Diabetic nephropathy is almost always associatedwith enlarged kidneys prior to terminal renal failure. Echogenicity increases with increasingstages of renal failure; however in earlier stages of disease echogenicity is frequently normal.A further diagnostic criterion is an increased RI (see below) however, the increased RI occursrelatively late in the course of diabetic nephropathy when other signs like microalbuminuriaare also already present. Even when terminal renal failure is present diabetic kidneys remainrelatively enlarged as opposed to kidneys from patients with glomerulonephritic or renalinterstitial disease. Diminished parenchymal width or a kidney with a low volume in a patientwith diabetes can hint at superimposed hypertensive nephrosclerosis. Renal scars, abscessesand papillary necrosis can also occur in diabetic nephropathy.
Glomerulonephritis and vasculitisThere is no specific ultrasonographic sign for glomerulonephritic disease. Depending on thedegree of renal functional impairment kidneys frequently lose volume and almost alwaysshow increased echogenicity but no renal scaring. Increased echogenicity is less notable inIgA nephropathy, minimal change disease and membranous glomerulonephritis andcorticomedullary differentiation is better preserved than in proliferative and interstitialglomerulonephritis. In acute glomerulonephritis kidneys are enlarged or of normal size, theparenchyma is frequently of increased depth and echogenicity but normal or even decreasedechogenicity can also occur. A renal segmental artery RI value of 0.80 or greater is a badprognostic sign 9. About 3-5 years after initiation of dialysis treatment secondary cystformation frequently occurs. These cysts may undergo malign transformation. For this reasonyearly control ultrasonography is indicatedFigure 7: Condensed and small right kidney in preterminal renal failure due to chronic IgAnephropathyHypertensive nephrosclerosisHypertensive nephrosclerosis is frequently associated with small and echo dense kidneys andparenchyma of diminished thickness. The renal resistive index is frequently elevated above0.80 and this is a bad prognostic sign.AmyloidosisEnlarged kidneys with thick parenchyma, greatly increased echogenicity, preservedcorticomedullary differentiation, medullary pyramids with low echogenicity and anincreased renal resistive index ( 0.7-0.8) are the unspecific hallmarks of renal amyloidosis.Acute renal failureIn acute intrarenal failure kidneys are frequently enlarged, parenchymal echogenicity isincreased, medullary pyramids appear to have low echogenicity and renal RI is severelyincreased. In acute prerenal failure renal echogenicity and renal RI is frequently normal.Patients with severely increased echogenicity appear to have a worse prognosis.
Hemolytic uremic syndromeDue to endothelial damage occlusion of small intrarenal vessels is the hallmark of thisdisease. This process is accompanied by an increased renal RI value ( 0.80). In addition tothe clinical presentation and laboratory parameters like elevated LDH, thrombocytopenia andfragmentocytes the increased RI can be used as a diagnostic clue.Hepatorenal syndromeAccording to B-mode ultrasonography the kidneys appear completely normal. However, arenal RI 0.70 in patients with liver cirrhosis and ascites is associated with a 20-30 foldincreased likelihood for the development of hepatorenal syndrome.Tubulointerstitial diseasesPyelonephritisRarely the renal pelvis is filled with echogenic material. As in all acute diseases the kidneysare enlarged. Due to affection of pararenal tissues renal motility is frequently impaired.Chronic pyelonephritis with impaired renal function is frequently associated with reducedrenal volume and renal scars. The scars traverse the parenchyma and pyelectasia is frequentlyseen next to a scar. Renal echogenicity is increased in a patchy pattern and thecorticomedullary differentiation is lost. Renal parenchymal width is diminished locally orgeneralized. Xantogranulomatous pyelonephritis is a variant, which in 2/3 of cases occurs dueto pelvic obstruction with infected renal stones (struvite stones magnesium-ammoniumphosphate). This variant results in a chronic, putrid and fatty inflammation with completedestruction of the renal architecture. The destructed area may be confused with renal tumorsor cysts. Renal stones or calcifications can frequently be found in affected areas. Definitivediagnosis relies on a CT-investigation(Medullary) nephrocalcinosis
Nephrocalcinosis is defined by deposits of calcium and phosphate in renal tubules.Ultrasonography can only depict calcification in the region of the terminal collecting ducts(medullary pyramids). Medullary pyramids normally have lower echogenicity in comparisonto the surrounding parenchyma and develop a hyper echoic ring or become completely hyperechoic sometimes even with acoustic shadowing. Classification in a 3 stage system has beensuggested. Stage 1 is depicted by a barely visible hyper echoic ring, stage 2 by a completering; both stages do not sho
Ultrasonography is used to determine the site and size of the kidney and to detect focal lesions like tumors, cysts and renal stones. Furthermore the presence and urodynamic relevance of hydronephrosis can reliably be found.
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